The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A typical ceramic heater generally includes a ceramic substrate and a resistive heating element either embedded within or secured to an exterior surface of the ceramic substrate. Heat generated by the resistive heating element can be rapidly transferred to a target object disposed proximate the ceramic substrate because of the excellent heat conductivity of ceramic materials.
Ceramic materials, however, are known to be difficult to bond to metallic materials due to poor wettability of ceramic materials and metallic materials. Many of the ceramic materials and the metallic materials are non-wetting, making it difficult to cause a molten metal to flow into the pores of a ceramic material against capillary pressure. Moreover, the difference in coefficient of thermal expansion between the ceramic material and the metallic material is great and thus a bond between the ceramic material and the metallic material is difficult to maintain at a high temperature.
Therefore, a thermocouple used with the ceramic heater is generally attached to the ceramic substrate through a metal sheath. The hot junction, or measuring junction, of the thermocouple for measuring temperature of the ceramic heater is received within and welded to the metal sheath, which in turn is secured to the ceramic substrate. The sheath is typically disposed in the proximity of the ceramic substrate by mechanical attachment, such as a spring loaded device.
This conventional method of securing the thermocouple to the ceramic heater has a disadvantage of delayed temperature response because the thermocouple measures the temperature of the metal sheath, rather than directly measuring the temperature of the ceramic substrate. Also the large thermal mass of the sheath tends to further delay the temperature change in the thermocouple. Therefore, an accurate temperature measurement by the thermocouple depends on the thermal characteristics of the metal sheath. When the ceramic heater is ramped at a very fast rate, the thermocouple may not accurately measure the temperature of the ceramic heater instantaneously if the metal sheath does not respond rapidly to the temperature change of the ceramic substrate. Accordingly, in a ceramic heater powered at a relatively high power density and ramped at a relatively fast rate, “overshooting” is likely to occur, which refers to an undesirable control of a parameter when the transition of the parameter from a lower value to a higher value exceeds the final value. Because of the inability to accurately measure and control the temperature over a ramping profile, the ceramic heater may be raised to a temperature exceeding the target temperature, resulting in an undesirable heating of the target object.